A charged particle beam imaging system uses a charged particle beam, called primary charged particle beam, to scan and impinge on the surface of a sample, and then collects the secondary charged particles emitted from the sample to form an image. For a given primary charged particle beam condition, the strength of secondary charged particle signal depends on surface topographical condition, material composition and local voltage distribution, thus the resulting image may reveal the surface topography, material contrast and/or voltage contrast.
A conventional charged particle beam imaging system generally includes a primary charged particle beam source which generates a charged particle beam, a condenser lens to condense the generated charged particle beam, an objective lens to focus the charged particle beam into a fine probe, a deflection unit to scan the fine probe over the sample held on a stage. The charged particle beam probe interacts with the sample to excite secondary charged particles which carry information of local topography, material, and potential of the sample. A detection unit collects these secondary charged particles to form an image of the sample. Proper sample surface charging condition is preferred for delivering a stable and uniform imaging of high contrast and high resolution. However, in some cases the charged particle beam itself may not be enough to create the preferred charging condition during imaging scanning, or the charged particle beam may induce severe surface charging which significantly deteriorates the image quality in the way of distortion, defocus, and/or lack of contrast for a neighboring subsequently imaged area. Thus, certain imaging methods with charging regulation before and after the imaging scanning, called pre-scan and post-scan respectively hereafter, have been developed and are important for the conventional charged particle beam imaging system.
Referring to FIG. 1, which is a charged particle beam imaging system 100 in accordance with the prior art. A primary charged particle beam is generated from a charged particle beam source 110 which may be, for example but not limited to, an electron beam gun. The primary charged particle beam is condensed by a condenser lens module 120 and focused by an objective lens module 130 to form a charged particle beam probe 140. A deflection unit 150 scans the charged particle beam probe 140 in lines across the surface of a sample 195 on a sample stage 190. It is noted that the one dimensional line scan can be converted to two dimensional raster by offsetting the beam center, or by moving the stage 190 properly in an orientation perpendicular to the line scan direction. After the bombardment of the charged particle beam probe 140 on the sample 195, secondary charged particles 160, for example but not limited to, secondary electrons, are emitted from the sample 195 and along with the backscattered charged particles, for example but not limited to, backscattered electrons, are collected by a charged particle detector 170. Since the amount of secondary charged particles are modulated by surface topography or voltage of scanned area, a two dimensional image representing the topography contrast or voltage contrast is obtained. The sample 195 may be a wafer, a mask or a semiconductor device and so on. It is noted that for simplicity of explanation, from here on the term charged particle “beam” and “beam probe” will be referring to the same beam which is defined as being substantially focused on the sample surface.
In general cases, charging will be induced on the scanned area by the primary charged particle beam and will accumulate if it cannot be released to ground quickly. Though proper level of charging of sample is preferred to deliver a voltage contrast image, excessive and none-uniform charging will result in adverse effects on image by distorting and/or defocusing the primary charged particle beam, so it is important to develop a method to regulate the surface charging initially existed or induced by previous imaging scanning.
There is an opposite situation where charging incurred by the primary charged particle beam during imaging scanning is too weak to produce an acceptable voltage contrast image, or the initial surface charging before image scanning is not uniform enough for providing an uniform image. Thus there is a need to have an additional step to regulate the surface charging condition, for example before the imaging scanning, and integrated into the image scanning process.
Currently, in some cases a general approach to regulate the sample surface charging condition is through a flood gun 180 to intentionally scan a separate charged particle beam, an optical beam or an electromagnetic radiation across the sample surface, so as to create a desired charging condition thereon. This operation is typically performed on a frame of image or wafer basis i.e. it treats the sample surface in a large area at one time. However, with such approach, when a subsequent imaging scanning operation is performed after the regulation, the established charging condition may have changed due to, for example, interaction of the regulated sample surface with the environment. In particular, the charging condition at the last regulated regions of the sample surface may have changed by the time the imaging scanning is performed thereto. As a result, the image quality of the obtained images may still not meet satisfaction.
Accordingly, a method is desired to control the surface charging condition of the sample in a more real time manner thereby rendering a more properly-controlled and uniform image quality.